1. Field of the Invention
The field of the invention is internal-combustion engines for motor vehicles.
2. Related Art
The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide. Accordingly, a need exists for a new approach which can significantly improve the efficiency of fuel utilization for automotive powertrains while still achieving low levels of NOx emissions.
Internal combustion engines create mechanical work from fuel energy by combusting the fuel over a thermodynamic cycle consisting (in part) of compression, ignition, and expansion. The efficiency with which mechanical work is converted from the available fuel energy is determined by the thermodynamic efficiency of the cycle. Thermodynamic efficiency, in turn, is determined in part by (a) the degree to which the fuel/air mixture is compressed prior to ignition (compression ratio), and (b) the final pressure to which the combusted mixture can be expanded while performing useful work on the piston which is related to the expansion ratio of the power or expansion stroke. Generally speaking, the lower the final pressure achieved during expansion against the piston, the greater the amount of work extracted. The pressure drop is limited by the fixed maximum volume of the cylinder, since there is only a finite volume available in which combusting gases may expand and still perform work on the piston. At some point the piston will reach bottom dead center, after which the gases, still at a high enough pressure to perform work, must be exhausted from the cylinder as the piston begins to rise again.
To fully utilize the pressure of the combustion gases, it would be necessary to expand the gases to ambient pressure while pushing against the piston. The phenomenon is illustrated in FIG. 1. Normally, gases are exhausted to the atmosphere when the expansion of the combustion cylinder stops. Some of the work extracted is represented by the unshaded area under the curve. The pressure of this exhausted gas is still higher than ambient pressure. If this residual pressure were expanded against another piston to ambient pressure, the additional work would equal the area represented by the shaded area under the curve. Some of this additional work ("A") would go toward operating the engine itself, but a significant amount ("B") would remain to create a net increase in work extracted.
Reaching such a low pressure would require a larger volume in which to expand the products of combustion, suggesting that the stroke of the piston or the maximum volume of the cylinder should be increased during the expansion stroke. Of course, the compression ratio would then increase in the same manner because the compression ratio is also governed by maximum cylinder volume. The result would be simply a larger engine cylinder, or an unacceptably large compression ratio.
Conventional engines are limited to having an expansion ratio roughly equal to the compression ratio. This is because compression and expansion both take place in a single cylinder that has a fixed maximum and minimum volume. It is possible to effectively change the two ratios relative to one another by manipulating the characteristics of the fuel-air mixture. For example, turbocharging and supercharging are used to increase the effective compression ratio relative to the expansion ratio. This is done by forcing a greater mass of air (and ultimately fuel/air mixture) into the combustion chamber without changing the actual volumetric compression ratio. This leads to increased power for a given engine displacement. But this approach does not affect the actual volumes involved and cannot provide a way to improve the expansion ratio relative to the compression ratio. Similarly, by restricting the flow of air into the cylinder during the intake stroke, or by other manipulation of exhaust or intake valves, it would be possible to reduce the effective compression ratio relative to the expansion ratio. However, this would introduce fluid-mechanical problems due to air flow and cylinder pressures that would probably require sophisticated timing strategies and detrimentally affect the efficiency of the thermodynamic cycle.
An engine design for increasing the expansion ratio relative to the compression ratio by means of dual cylinder expansion, is disclosed in a 1993 paper published by the Society of Automotive Engineers (SAE number 930986). The disclosed design includes an auxiliary cylinder dedicated to further expansion of gases against a piston after they have been exhausted from the main combustion cylinders. The system also includes a compression cylinder to provide supercharging capability. However, the valving arrangements of this system would require two additional valves per cylinder, one for supercharging and one for expanding, for a total of four valves per combustion cylinder. In addition, the design disclosed in this SAE paper utilizes two valves each, for the separate expansion and companion cylinders. The configuration as shown requires long runners between the combustion cylinders and the auxiliary cylinders, which runners would increase the effective expansion volume, introduce pressure losses, and possibly introduce back-pressure problems that would require complex valving and control to overcome. Its main purpose seems to be to improve power output rather than reduce NOx emissions and improve energy conversion efficiency, as indicated by an integrated supercharging device.